1. Field of the Invention
This invention relates to waste management and, more particularly, to the disposal of radioactive waste containing bead ion exchange resins. In one of its more particular aspects, this invention relates to a process for reducing the volume of bead ion exchange resin wastes. In another of its more particular aspects, this invention relates to a process for disposing of bead ion exchange resin wastes in the form of solid monoliths.
2. Prior Art
Waste management frequently involves the necessity of disposing of large volumes of materials, some of which may be contaminated with hazardous substances. In nuclear power plants, for example, large amounts of radioactive liquid and solid wastes, known as low-level radioactive wastes, are produced. Low-level radioactive wastes differ from high-level radioactive wastes, which are produced in the reprocessing of nuclear fuels, in that the latter represent greater risks of contamination and, therefore, require disposal techniques which are more stringent than in the case of low-level radioactive wastes. Disposal of radioactive wastes, in general, cannot be readily accomplished by using conventional waste disposal techniques because of the relatively long half-lives of certain radioactive elements. The most widely used disposal techniques for radioactive wastes are storage, solidification, and burial. The expense of so disposing of large volumes of radioactive wastes, however, is constantly rising and approaching levels at which volume reduction becomes economically desirable.
Many different methods for disposing of radioactive wastes are known.
U.S. Pat. No. 3,101,258 describes using a heated wall spray calcination reactor for disposing of nuclear reactor waste solutions. In spray calcination reactors of the heated wall type, however, the temperature gradient from the outside of the reactor inward may result in uneven heating, producing regions of undesired high temperatures and causing non-uniform results.
U.S. Pat. No. 3,922,974 discloses using a hot air-fired furnace for incinerating radioactive wastes. The use of this apparatus, however, results in the production of noxious off-gases, necessitating additional processing for removal of such gases.
U.S. Pat. No. 4,145,396 describes the volume reduction of organic waste material contaminated with at least one volatile compound-forming radioactive element selected from the group consisting of strontium, cesium, iodine, and ruthenium. The selected element is fixed in an inert salt by introducing the organic waste and a source of oxygen into a molten salt bath maintained at an elevated temperature to produce solid and gaseous reaction products. The molten salt bath is comprised of one or more alkali metal carbonates and may optionally include about 1-25 wt % of an alkali metal sulfate. Although effective in reducing the volume of certain organic wastes, the separation of the radioactive components from the nonradioactive components of the molten salt bath requires a number of additional processing steps.
In U.S. patent application Ser. No. 451,516 filed Dec. 20, 1982, now U.S. Pat. No. 4,499,833, and assigned to the assignee of the present invention, there is described a process for converting radioactive wastes in the form of liquids, solids, and slurries into a mixture of a nonradioactive gas and a radioactive inorganic ash. In accordance with that process, the radioactive waste is introduced as a finely atomized spray into a zone heated by means of a hot gas to a temperature sufficient to effect the desired conversion, preferably a temperature in the range of about 600.degree. to 850.degree. C. The process is conducted in a spray dryer modified to combust or calcine the waste.
While the foregoing patent application discloses a process which is satisfactory for destroying radioactive wastes, the high temperatures utilized in the process can produce noxious gases such as NO.sub.x or SO.sub.x, the removal of which necessitates taking additional measures to ensure that any gas ultimately released to the atmosphere is non-polluting. In addition, such high temperatures specifically lead to the ignition and combustion of organic materials in the waste.
Consequently, there is a need for a process which can be used to reduce the volume of radioactive wastes without producing noxious off-gases or combusting organic materials present in the wastes. This need is particularly pronounced in the case of liquid low-level radioactive wastes where large volumes of wastes of relatively low radioactivity compound the problems and costs involved in their transportation and disposal.
In U.S. patent application Ser. No. 467,272 filed Feb. 17, 1983, and assigned to the assignee of the present invention, there is disclosed a process for reducing the volume of low-level radioactive wastes by removing the free water contained in the waste by means of spray drying at a temperature sufficient to vaporize the water contained in the waste but insufficient to oxidize the waste or to volatilize any radionuclides present in the waste, preferably a temperature in the range of about 65.degree. to 205.degree. C. The process comprises introducing the waste as a finely atomized spray into a zone heated by means of a hot gas. In this process, contact of the waste particles sprayed into the zone with the hot gas results in the production of a dry, flowable, radioactive solid product and a gaseous, relatively nonradioactive product which contains substantially no NO.sub.x or SO.sub.x and little, if any, volatile radionuclides, which are retained in the solid product. This process is particularly suitable for low-level radioactive wastes such as sodium sulfate slurries, boric acid slurries, and powdered ion exchange resins. In the case of ion exchange resins, however, while this process is capable of drying powdered ion exchange resins, which have a mean diameter in the range of about 50 to 60 microns, bead ion exchange resins, which have a mean diameter in the range of about 500 to 800 microns, are only partially dried by means of this process. In fact, only the free water on or near the surface of the ion exchange resin beads is removed, leaving behind much of the water contained inside the ion exchange resin beads, which constitutes a major proportion of the volume and weight of the ion exchange resin beads.
It is desirable in the processing of various waste materials, and especially radioactive wastes, to reduce the volume of the waste in order to minimize the costs of disposal. Removing the water which is contained inside ion exchange resin beads would significantly reduce the volume of the beads and provide a product which could be disposed of with facility. However, to date no process has been found effective to accomplish this purpose without completely destroying the ion exchange resins and producing oxidation products and noxious gases which further complicate the disposal process.